Recent years have seen an increasing number of cases where a composite material such as fiber reinforced resin (FRP) is used as a structural member of a machine structure. A representative example is blades assembled to a wind turbine unit. The wind turbine unit is an energy conversion device employed for power generation by wind. At least two blades are assembled to a rotor which is directly connected to a generator or indirectly connected thereto via a gear box or hydraulic power unit. The rotor is rotated by wind power acting on the blades while the rotational energy of the rotor is converted to an electric power by the generator. A housing part accommodating the generator and the like is called a nacelle which is commonly assembled to a tower. The wind turbine unit of such a structure is well known in the art. In particular, the blades are generally made of FRP excellent in specific rigidity and specific strength. The FRP as the structural member is finding application as, for example, vehicle body of automobile or railway vehicle besides the blades of the wind turbine unit. These structural members are exposed to repeated load application and hence, must be designed after ensuring sufficient fatigue strength. It is known that glass fiber and carbon fiber per se, which are used as a reinforcing fiber for FRP, sustain very little fatigue damage. However, a resin material used as a matrix sustains fatigue damage caused by repeated load application. As a result, the FRP suffers fatigue damage. Therefore, sufficient consideration should be given to the fatigue strength of FRP when designing a structure of the above product. However, the above products are used in a number of different environments. This makes it no easy task to make an accurate estimation of load on the structural member at the time of designing. In the case of wind turbine unit, for example, the load on the blades is assumed to vary significantly depending upon wind conditions to which the wind turbine unit is exposed. In fact, it is impracticable to accurately estimate the wind conditions over the entire life of the wind turbine unit. For accurate assessment of the load on the blades, it is effective to make a direct measurement of stress history by means of a strain gauge attached to the blade, as suggested by Patent Literature 1.
On the other hand, there are known several methods of directly assessing a damage state of the composite material without relying on the load history. These methods include, for example, a technique of using an AE sensor for capturing an AE wave emitted from internal damage of a material, and a technique of using supersonic flaw detection, and the like. As for the blades of wind turbine unit, for example, Patent Literature 2 discloses a technique in which an acceleration sensor is attached to the blade so as to determine a natural frequency of the blade based on the frequency characteristic of the blade and the soundness of the blade is determined based on change in the natural frequency.